1. Field of the Invention:
The present invention relates to heat exchange vessels, such as nuclear steam generating vessels, and particularly to control of the flow of secondary fluid around the outsides of heat exchange tubes and management of the deposition of sludge on the tube sheet of such a heat exchanger.
2. Description of the Prior Art:
A typical nuclear steam generator comprises a vertically oriented shell or vessel and a plurality of inverted U-shaped tubes disposed in the shell so as to form a tube bundle. Each tube has a pair of elongated vertical portions interconnected at the upper end by a curved bight portion, so that the vertical portions of each tube straddle a center lane or passage through the tube bundle. A tube sheet supports the vertical portions of the tubes at their lower ends. In some steam generator vessels, the tube sheet is "stayed" from beneath by a central support post and has no tubes in the central region overlying the support post. The upper surface of the tube sheet may have a shallow cavity in this central region.
The vertical tube portions on one side of the center lane communicate with a primary fluid inlet header beneath the tube sheet and form the "hot leg" of the tube bundle, and those on the other side of the center lane communicate with a primary fluid outlet header beneath the tube sheet, forming the "cold leg" of the tube bundle. The steam generator also comprises a cylindrical wrapper sheet disposed between the tube bundle and the shell, cooperating with the shell to form an annular downcomer chamber, terminating a predetermined distance above the tube sheet.
The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet header, is transmitted through the tube bundle and out through the primary fluid outlet header. At the same time, a secondary fluid or feedwater is circulated around the tubes above the tube sheet in heat transfer relationship with the outside of the tubes, so that a portion of the feedwater is converted to steam, which is then circulated through standard electrical generating equipment. More particularly, the feedwater is conducted down the annular chamber along the outside of the wrapper and to the tube sheet, radially inwardly along the tube sheet and upwardly among the tubes inside the wrapper.
The feedwater contains particles of material, mainly in the form of iron oxides and copper compounds along with traces of other metals, which tend to settle out of the feedwater onto the tube sheet in those areas of the tube sheet where the velocity of lateral flow across the tube sheet is insufficient to prevent settling. The settling is harmful because it creates buildups of sludge deposits which provides sites for concentration of corrosive agents at the tube walls that result in tube corrosion.
It is inevitable that regions of low lateral or radial velocity across the tube sheet will be formed in the tube bundle. In order to minimize damage caused by sludge build-up in such low velocity areas, it is desirable to localize this buildup to the regions of the tube sheet where there are no tubes, e.g., along the center tube lane or at the untubed central region of the tube sheet. Thus, it is desirable to control the flow of secondary fluid so that the regions of low lateral or radial velocity occur in these regions without tubes. An optimal design is one which yields the smallest low velocity area and, at the same time, locates it at the center of the tube sheet.
The flow pattern of the secondary fluid is affected by a number of factors. As the feedwater enters the tube bundle from beneath the tube wrapper, the radial inward flow along the tube sheet is impeded by the tubes. Since there are no tubes along the tube lane, there tends to be relatively high flow velocity therealong. To minimize tube lane flow velocity, there have been provided tube lane blocks at spaced-apart locations along the center lane to inhibit the flow of feedwater therealong, and thereby increase the flow velocity in the regions of the tube sheet where there are tubes. But this does not serve to eliminate the presence of other low velocity regions in the tube bundle.
Heat transfer rate in the "hot leg" in the tube bundle is about 4 to 5 times that in the "cold leg" in the vertical region between the tube sheet and the first of the several tube support plates. This results in boiling of the feedwater in this region of the "hot leg", generating vapors. The buoyancy force associated with the steam vapors can pull the feedwater in the "cold leg" toward the "hot leg", a phenomenon known as "thermal siphon". This "thermal siphon" causes a low velocity zone of the secondary fluid flow to appear in the middle of the "hot leg" part of the tube bundle. Attempts have been made to counteract this "thermal siphon" effect by a technique known as "feedwater offset", whereby a larger fraction of the total feedwater is discharged into the downcomer path on the "hot leg" side of the vessel than on the "cold leg" side. The resulting greater cooling of the water may be maintained throughout the downcomer path and, thus, at the wrapper inlet, thereby significantly reducing "hot leg" boiling and thereby suppressing the "thermal siphon" effect. But, this technique has been ineffective because of a swirling motion which exists within the downcomer path.
When the feedwater enters the tube bundle through the opening at the bottom of the wrapper, the radial inlet flow has a tendency to immediately turn upward, since the flow resistance in the vertical direction, parallel to the tubes, is much less than that in the lateral or radial direction. This results in weak penetration of the radial flow into the tube bundle in the vicinity of the tube sheet. Therefore, means have been utilized to enhance radial flow velocity by reducing vertical flow velocity. To this effect, a flow distribution baffle plate has been utilized a predetermined distance above the tube sheet, to provide additional resistance to vertical axial flow, and thereby promote lateral flow penetration into the bundle. But, this has been insufficient to move the low velocity or stagnation areas to the center of the tube sheet.